subscapularis

**Anatomy** - Origin: subscapular fossa (anterior surface of the scapula). - Insertion: lesser tubercle of the humerus. The upper portion is tendinous and the lower portion is muscular. - Action: internal (medial) rotation of the arm. It also adducts the arm and stabilizes the humeral head in the glenoid, which resists anterior dislocation. - Innervation: upper and lower subscapular nerves (C5–C6), branches of the posterior cord of the brachial plexus. The lower subscapular nerve also supplies teres major. - Blood supply: subscapular artery (a branch of the axillary artery) and the dorsal scapular and suprascapular arteries. - Rotator cuff mnemonic: SITS (Supraspinatus, Infraspinatus, Teres minor, Subscapularis). Subscapularis is the only one that internally rotates. The others are: - Supraspinatus: initiates abduction (first 0–15°). - Infraspinatus and teres minor: external rotation. - Subscapularis is the only rotator cuff muscle that inserts on the lesser tubercle. The other three insert on the greater tubercle. - Its position anterior to the scapula, between the scapula and the serratus anterior, forms part of the posterior wall of the axilla.

**Clinical significance** - Subscapularis tear or injury: usually from traumatic anterior shoulder dislocation, forceful hyperextension or external rotation, or chronic degeneration. It is the least commonly torn rotator cuff muscle. The supraspinatus is the most commonly injured. - Exam findings: weak internal rotation and an increased passive external rotation range. - Special tests: - Lift-off test (Gerber): the patient cannot lift the dorsum of the hand off the lower back. - Belly-press (Napoleon) test: the patient cannot keep the elbow forward and presses the abdomen by flexing the wrist instead of internally rotating at the shoulder. - Bear-hug test. - Long head of biceps tendon: it can sublux or dislocate medially when the subscapularis tendon or transverse humeral ligament is torn. The tendon passes in the bicipital groove between the greater and lesser tubercles. - Anterior shoulder dislocation: the subscapularis acts as an anterior static and dynamic stabilizer. Damage to it is associated with recurrent instability. The axillary nerve can also be injured in anterior (inferior) dislocation. It runs with the posterior circumflex humeral artery through the quadrangular space. - Nerve injury: upper trunk (Erb) lesions affect C5–C6, which can weaken subscapularis along with deltoid, supraspinatus, and infraspinatus. This produces the "waiter's tip" posture, with the arm adducted and internally rotated. - Imaging: MRI is the best test to assess partial or complete rotator cuff tears. - Management: partial tears and mild cases are treated with rest, NSAIDs, and physical therapy. Full-thickness tears in active patients, or those that fail conservative treatment, are repaired surgically.

**High-yield pearls** - The subscapularis is the largest and strongest cuff muscle. - It is the only cuff muscle on the anterior scapular surface. - Think "lesser tubercle, internal rotation, lift-off test." - Remember that the upper and lower subscapular nerves are different from the suprascapular nerve (C5–C6, upper trunk), which supplies the supraspinatus and infraspinatus.